The use of antibiotic prophylaxis for the prevention of endocarditis in patients at risk is well established. Dentists' knowledge of the dynamics of this disease and its prevention is important: because invasive procedures producing bacteremia are presumed to be precipitating causal events in the genesis of endocarditis; because of the large population at risk; because the disease is potentially life-threatening; and because those who survive suffer lifelong morbidity. The research design is grounded in our previous research findings. Because they have been found to have the lowest knowledge level, the target group for experimental interventions, designed to increase knowledge about management of patients at risk for bacterial endocarditis, is GPs 40 + years of age who are in solo practice. The two planned experimental interventions are also data based. 1) A version of the American Heart Association (AHA) recommendations; 2) Intervention (1) plus the most critical explanator of this knowledge, i.e., the Theory upon which the AHA recommendations are based. Both interventions will be a) designed for maximum impact by experts in the biomedical and audio-visual fields; b) mailed to respondents "packaged" in a laminate form, less likely to be discarded. The ultimate goal of our ongoing research has always been to improve the knowledge of American dentists about the management of patients at risk for bacterial endocarditis. Leading directly to this final goal, and associated with it, are three research goals (1) measurement of the effects, on the knowledge level, of the planned interventions, (2) measurement of the rate of attenuation of these effects, (3) measuring the rate of diffusion of the AHA recommendations. The research design for this study is a blend of 1) random sampling, in order to maximize the representativeness of the sample, and 2) random assignment in order to minimize threats to the internal validity of the study's conclusions. Subjects will be randomly assigned to six experimental conditions in order to optimally evaluate 1) the effect of alternative educational interventions utilizing planned T-Test comparisons with a Bonferroni adjustment to constrain the experiment-assigned error rate to no greater than p greater than or equal to .05/2; 2) the attenuation of experimental effects utilizing the same method to control the Type 1 error rate; 3) the diffusion of knowledge using a single T-Test.